The present invention relates to the art of nuclear medicine and diagnostic imaging. It finds particular application in localizing a scintillation event in a gamma camera having a number of photomultipliers arranged over a camera surface. It is to be appreciated that the present invention may be used in conjunction with positron emission tomography (xe2x80x9cPETxe2x80x9d), single photon emission computed tomography (xe2x80x9cSPECTxe2x80x9d), whole body nuclear scans, transmission imaging, other diagnostic modes and/or other like applications. Those skilled in the art will also appreciate applicability of the present invention to other applications where a plurality of pulses tend to overlap, or xe2x80x9cpile-upxe2x80x9d and obscure one another.
Diagnostic nuclear imaging is used to study a radio nuclide distribution in a subject. Typically, one or more radiopharmaceutical or radioisotopes are injected into a subject. The radiopharmaceutical are commonly injected into the subject""s bloodstream for imaging the circulatory system or for imaging specific organs that absorb the injected radiopharmaceutical. A gamma or scintillation camera detector head is placed adjacent to a surface of the subject to monitor and record emitted radiation. Each detector typically includes an array of photomultiplier tubes facing a large scintillation crystal. Each received radiation event generates a corresponding flash of light (scintillation) that is seen by the closest photomultiplier tubes. Each photomultiplier tube that sees an event generates a corresponding analog pulse. Respective amplitudes of the pulses are generally proportional to the distance of each tube from the flash.
A fundamental function of a scintillation camera is event estimation, which is the determination of energy and position of the location of an interacting gamma or other radiation ray based on the detected electronic signals. A conventional method for event positioning is known as the Anger method, which sums and weights signals seen by tubes after the occurrence of an event. The Anger method for event positioning is based on a simple first moment calculation. More specifically, the energy is typically measured as the sum of all the photomultiplier tube signals, and the position is typically measured as the xe2x80x9ccenter of massxe2x80x9d of the photomultiplier tube signals.
Several methods have been used for implementing the center of mass calculation. With fully analog cameras, all such calculations (e.g., summing, weighting, dividing) are done using analog circuits. With hybrid analog/digital cameras, the summing and weighting are done using analog circuits, but the summed values are digitized and the final calculation of position is done digitally. With xe2x80x9cfully digitalxe2x80x9d cameras, the tube signals will be digitized individually. In any event, because the fall-off curve of the photomultipliers is not linear as assumed by the Anger method, the image created has non-linearity errors.
One important consideration is the location of the event estimation. The scintillation light pulse is mostly contained within a small subset of the tubes on a detector. For example, over 90% of a total signal is typically detected in seven (7) out of a total number of tubes, typically on the order of 50 or 60. However, imaging based only on the seven (7) closest tubes, known as clustering, has poor resolution and causes uniformity artifacts. Furthermore, because the photomultiplier tubes have non-linear outputs, the scintillation events are artificially shifted toward the center of the nearest photomultiplier tube.
For a given detector geometry, the fall-off curve varies with a depth that a gamma photon interacts in the crystal. Different energy photons have varying interaction depth probabilities that are more pronounced in thicker crystals, which are typically used in combination with PET/SPECT cameras.
Therefore, separate linearity or flood correction tables are created and used for each energy in order to correct for the uniformity artifact. Fall-off curves are acquired using a labor intensive method of moving a point source a small amount (e.g., 2 mm) roughly 30-40 times for each tube. The individual tube""s output is acquired at each location, the mean value of the tube""s output is found, and a curve of tube output versus distance from the location of the point source is generated.
A disadvantage of generating a fall-off curve using a point source is the large amount of time required to move the source position. This method is also prone to errors in positioning the source accurately on the detector. It is also usually only done in one or two directions. Therefore, the assumption is made that the fall-off curve is exactly symmetric. Regenerating the fall-off curve for a different energy requires that the process be repeated again. Likewise, generating the fall-off curve for a different tube requires the process be repeated again. Therefore, the assumption is usually made that the fall-off curve is invariant across different detectors or photomultiplier tubes.
Generating the linearity correction tables typically involves using a lead mask that contains many small holes to restrict the incident location of radiation on the crystal surface. The holes represent the true location of the incident photons that interact in the detector crystal. This information is used to generate a table that consists of x and y deltas that when added to the x and y estimate, respectively, are used to generate a corrected position estimate that more accurately reflects the true position. A disadvantage is that new tables must be generated for each energy that is to be imaged, thereby increasing the calibration time. Another disadvantage is that the calibration mask has a limited number of holes, since each must be resolved individually, thereby limiting the accuracy of the correction. It is also increasingly more expensive and difficult to calibrate for higher energy photons since the thickness of the lead mask must increase in order to have sufficient absorption in non-hole areas.
Another prior art method uses separate flood uniformity correction tables for each energy. A disadvantage is that new tables must be generated for each energy that is to be imaged, which increases calibration time. Flood correction has the disadvantage of creating noise in the image, since the method is based on either adding or removing counts unevenly throughout the pixel matrix. This method is also sensitive to drift in either the photomultiplier tubes or electronics.
Another prior art method reduces the output from the closest tube. For example, an opaque dot is sometimes painted over the center of each photomultiplier tube. The sensitivity can also be reduced electronically. Unfortunately, the closest photomultiplier tube typically has the best noise statistics. Reducing its sensitivity to the event causes a resolution loss.
Similarly, excluding the outlying tubes reduces the noise in the determined values of energy and position. The most common way of excluding signals from outlying tubes includes imposing a threshold, such that tube signals below a set value are either ignored in the calculation or are adjusted by a threshold value. This method works reasonably well in excluding excess noise. However, the method fails if stray signals exist above the threshold value. Stray signals may exist at high-counting rates, when events occur nearly simultaneously in the crystal. When two events occur substantially simultaneously, their xe2x80x9ccenter-of-massxe2x80x9d is midway between the twoxe2x80x94where no event actually occurred. Nearly simultaneously occurring events may result in pulse-pile-up in the energy spectrum and mispositioning of events. This behavior is especially detrimental in coincidence imaging, where high-count rates are necessary.
Thus, it is desirable to improve localization in event estimation. With a fully digital detector, both the intensity and the location of each tube signal are known. It is, therefore, possible to calculate the energy and position based primarily on the tube signals close to an individual event. One current method for event localization is seven (7) tube clustering in which a cluster of seven (7) tubes is selected for each event. These tubes include the tube with maximum amplitude, along with that tube""s six (6) closest neighbors. This method is an effective method for limiting the spatial extent of the calculation. However, the main drawback of this method is the resulting discontinuity.
Discontinuity arises when the detected positions for events from a uniform flood source form an array of zones around each possible cluster. Elaborate correction schemes (see e.g., Geagan, Chase, and Muehllehner, Nucl. Instr. Meth. Phys. Res A 353, 379-383 (1994)) are needed to xe2x80x9cstitchxe2x80x9d together these overlapping zones to form a single, continuous image. However, this correction is sensitive to electronic shifts, which often arise in high-count situations, causing seam artifacts in the camera response.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.
A nuclear camera system includes a detector for receiving radiation from a subject in an exam region. The detector head includes a scintillation crystal, which converts radiation events into flashes of light, and an array of sensors, which are arranged to receive the light flashes from the scintillation crystal. Each of the sensors generates a respective sensor output value in response to each received light flash. A processor determines when each of the radiation events is detected. At least one of an initial digital position and an energy of each of the detected radiation events is determined in accordance with respective distances from a position of the detected event to the sensors. An image representation is generated from the digital positions.
In accordance with one aspect of the invention, each of the sensors is electrically connected to at least one of a plurality of analog-to-digital converters for converting the sensor output values from analog values to respective series of digital sensor output values.
In accordance with another aspect of the invention, the processor weights the sensor output values with weighting values for determining corrected positions of the events. The weighting values are determined in accordance with the respective distances from the position of each event to each of the sensors that detects the event.
In accordance with a more limited aspect of the invention, the processor determines a subsequent set of weighting values as a function of the corrected positions and energies of the events.
In accordance with another aspect of the invention, the processor generates the weighting values for each of the distances as a function of a desired response curve and an input response curve.
In accordance with a more limited aspect of the invention, the processor generates the weighting values as a function of the energy being imaged.
In accordance with an even more limited aspect of the invention, the processor generates energy ratio curves representing respective relationships between a plurality of the energies being imaged. The processor generates an energy scaling curve representing a relationship between the plurality of energies being imaged and respective scaling factors. Also, the processor generates the weighting values as a function of one of the scaling factors.
In accordance with another aspect of the invention, a look-up table is accessed by the processor for storing the weighting values.
In accordance with a more limited aspect of the invention, the look-up table is multi-dimensional and indexed as a function of at least one of time, temperature, count-rate, depth of interaction, and event energy.
In accordance with another aspect of the invention, the processor analyzes the sensor output values for detecting a start of the event.
In accordance with a more limited aspect of the invention, the processor analyzes the sensor output values for detecting a previous event. Any sensor output values associated with the previous event are excluded from calculations of an initial position and an energy of a next detected event.
In accordance with another aspect of the invention, in response to the processor detecting a next event after an integration period of the event begins, during which the position of the detection event is determined, the sensor values associated with the sensors of the next event are nulled from calculations of the initial position and the energy of the event.
In accordance with another aspect of the invention, a second detector disposed across an imaging region from the first detector. A coincidence detector is connected with the first and second detectors for detecting concurrent events on both detectors. A reconstruction processor determines rays through the imaging region between concurrent events and reconstructs the rays into an output image representation.
In accordance with another aspect of the invention, an angular position detector determines an angular position of the detector around an imaging region. A reconstruction processor is connected with the detector and the angular position detector for reconstructing a volumetric image representation from the corrected positions of the events on the detector and the angular position of the detector during each event.
In accordance with another aspect of the invention, the sensors include photomultiplier tubes.
One advantage of the present invention resides in its high linearity. Therefore, linearity and uniformity corrections are reduced.
Another advantage resides in improved accuracy in event positioning, even in high count and pile-up situations.
Another advantage is that local centroiding is continuous and seamless.
Another advantage resides in more accurate estimation of events.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.